The present disclosure relates to control of a gas turbine engine, and more specifically, to methods and systems for controlling a transfer between combustion modes while a gas turbine engine is under combustion direct boundary control.
In at least some known gas turbine systems, a type of control referred to as combustion direct boundary control is used to regulate flows of fuel and air to the various nozzles of the one or more combustors within a gas turbine engine. As used herein, “combustion direct boundary control” refers to the regulation of combustion within a gas turbine engine, e.g., via controlling the flows of air and/or fuel to nozzles in combustors within the engine, such that one or more predetermined combustion parameters, including but not limited to, temperatures, pressures, dynamics, and/or concentrations of combustion byproducts, are maintained within predetermined boundaries or limits. In at least some known gas turbine systems, the gas turbine engine includes a plurality of combustors, and each combustor includes a plurality of nozzles. In at least some such gas turbine systems, the flows of fuel and/or air are supplied to the individual nozzles via individual fuel and air supply circuits that can be controlled independently of each other. One way the amounts of air and/or fuel that are supplied to an individual nozzle within a combustor can be defined is by determining the total air and/or fuel required to be delivered to the gas turbine engine or combustor, then defining the amounts of fuel and/or air supplied to individual nozzles (sometimes referred to as “splits”). The “split” for a given nozzle defines the fraction of the total required fuel flow to the gas turbine engine or combustor to be delivered via the given nozzle. Accordingly, during combustion direct boundary control, a split being channeled to one nozzle may be different than a split being channeled to another nozzle within the same combustor.
During combustion direct boundary control, a control system for a gas turbine system uses one or more closed-loop feedback loops to adjust the splits supplied to the nozzles. Each loop can be defined by a boundary parameter. Boundary parameters can include, but are not limited to, predetermined numerical values or ranges for gas turbine emissions such as NOx, CO, etc., combustion system dynamics, and/or any combustor operability characteristics, including parameters indicative of lean blowout. For each feedback loop, the commanded split output is a function of a defined limit or target for the boundary parameter, feedback on the current value of the boundary parameter, and the current split. The feedback on the current boundary parameter can be obtained via a direct measurement of the boundary parameter, a modeled estimate of the boundary parameter, or a combination of both. Further priority logic can further downselect from a plurality of feedback loop splits to define the final commanded split for the given nozzle.
In at least some known gas turbine systems, the gas turbine engine is capable of being operated in several different combustion modes. The different combustion modes, which for purposes of this disclosure may be identified via numbers (1, 2, 3, etc.) and/or letters (A, B, C, . . . , X, Y, Z, etc.), are differentiated from each other with respect to the amounts of fuel and/or air supplied to each nozzle within a combustor and/or with respect to the amounts of fuel and/or air supplied to the respective combustors within the gas turbine engine. More specifically, the different combustion modes determine which nozzles will be enabled (i.e., supplied with some amount of fuel and/or air), and which nozzles will be disabled (i.e., not supplied with fuel and/or air).
In at least some known gas turbine systems, the different combustion modes may be required to operate the gas turbine engine optimally across a range of operating conditions. This range of operating conditions includes different loading conditions imposed on the gas turbine engine. Accordingly, one combustion mode may correspond to a low load mode, another combustion mode may correspond to a mid-load mode, and still another combustion mode may correspond to a high load mode. These are examples only, and in at least some known gas turbine systems, different combustion modes may be associated with other criteria of concern. In addition, there may exist secondary factors that may affect the choice of combustion mode for a given gas turbine operating condition, such as ambient conditions, gas turbine conditions, etc.
In such gas turbine systems that are capable of operation in multiple combustion modes, it may be desirable to transfer from one combustion mode to another while the gas turbine engine is being continuously operated. However, in such known gas turbine systems, it has not been possible to transfer between combustion modes while still operating under combustion direct boundary control conditions. This is because some of the boundary measurements used directly for control and/or as input into the models for unmeasured boundary parameters used for control are dependent on the current combustion mode. As such, these measurements cannot be used to accurately predict the passive splits. Therefore, it is necessary for the control system to exit combustion direct boundary control and refer to an open loop split schedule without direct feedback loops. Open loop splits can be scheduled based on a measured or modeled combustion reference parameter, one that is not dependent on the combustion mode, and will usually be stored in memory within the control system, that contain splits that correspond to various combustion modes under different operating conditions in order to implement a change between combustion modes. After initial or “landing spot” splits have been retrieved from a split schedule, and the related measurements fully reflect the gas turbine operating in the new combustion mode, then combustion direct boundary control operation of the gas turbine engine can be resumed. However, this method has several disadvantages. First is the lack of robustness in running to open loop schedules and added risk of potentially violating a boundary limit when direct boundary control is disabled. Secondarily, determining and maintaining numerous split schedules for different combustion modes and different conditions (e.g., different load paths or exhaust temperatures) for each mode may be labor-intensive and expensive, as the generation and maintenance of predetermined split schedules for a gas turbine engine involves the use of maintenance personnel for repeated periodic onsite tuning of the gas turbine engine.